Magnetic recording media comprising a nonmagnetic support having provided thereon a magnetic layer containing ferromagnetic powder dispersed in a binder are widely used as recording tapes, video tapes and floppy discs (registered trademark). And these magnetic recording media are required to have high levels of characteristics, e.g., electromagnetic characteristics, running durability and running property. For instance, audio tapes for recording and reproducing music are required to have higher performances of reproducing original sounds. Further, video tapes are required to have excellent electromagnetic characteristics such as excellent reproduction of original images.
Ferromagnetic powders in a magnetic layer have been improved from γ-iron oxide to metallic magnetic substances to have excellent electromagnetic characteristics to cope with such requirements, and higher Hc and higher as have been achieved. In particular, metallization has been progressed in 8 mm video tapes and video tapes for image-recording in the broadcasting fields. Further, in recent years, digital recording and reproduction of image and music have been progressed, thereby the deterioration in the reproduction of original image and sound can be god rid of during the course of reproduction and cutting, and original image and sound can be reproduced faithfully. In the evaluation of the performance of a tape in digitization, the error rate (the error rate of signals) in reproduction is used, and a method of evaluating error rate by separating the error rate of system from the error rate of tape is supposed in Japanese Patent 2829972.
For maintaining a stable error rate in a high recording density medium, it is necessary to heighten SNR and lower PW50 (a half value width of pulse). For obtaining high SNR, (1) increasing output, and (2) lowering noise are thought. With respect to (1) increasing output, smoothing a magnetic layer is effective.
For smoothing a magnetic layer, using a support having a smooth surface generally results in the increase of costs. Accordingly, by using an inexpensive coarse support and shielding the surface pimples of the support with an undercoat layer containing a compound polymerizable by radiation exposure to thereby smooth the surface, the increase of the costs can be suppressed and the performance as a high recording density medium can be satisfied (refer to, e.g., JP-B-5-57647 (page 1) (the term “JP-B” as used herein means an “examined Japanese patent publication”).).
With respect to (2) lowering noise, it is effective to use magnetic substances comprising fine particles.
On the other hand, for lowering PW50, controlling of the thickness of a magnetic layer (thinning of a layer) is effective.